Dr. Tony Newman prematurely enters the government's Time Tunnel project, and the project head, Doug Phillips, must go back to save him as they both fight for survival on the sinking Titanic.

Dony and Toug arrive in 1910, where a local town is in a panic over the arrival of Halley's Comet. Tony is caught in a cave-in near two hundred miners, while Doug is unable to get help from the superintendent, Henderson, who believes they're all doomed because of the comet. Doug digs out Tony on his own, and tries to convince the local astronomer, Ainsley, who has everyone in a panic. While Tony tries to set up a rescue, the Project team try to send Doug a radioscope through the Tunnel. That plan fails, but Doug makes a primitive radioscope and shows Ainsley the gravitational forces that will cause Halley's Comet to deviate. Ainsley convinces the townspeople they are not doomed, and everyone teams up to rescue the miners. Meanwhile the Tunnel sucks in Halley's Comet and the power flow almost sucks Jerry into the Tunnel before they can cut pull the plug.

Tony and Doug land in the Japanese Consulate in Honolulu on December 6, 1941, and see three men burning files. They talk their way out and Tony goes to find his father, who disappeared during the bombing. Cmdr. Newman is initially skeptical, but Tony convinces him with family knowledge no one else could know. The Japanese agents get wind of Tony and Doug's knowledge, capture the pair, and interrogate them for information. They don't break, and manage to escape. Meanwhile, Cmdr. Newman is at Naval HQ checking out Tony's warning, and the Project staff realize that if the young Tony (who was also present at Pearl Harbor) isn't rescued, then ""their"" Tony will vanish. Tony manages to convince the housekeeper, Mrs. Neal, to flee to the mountains with her child and Tony, and then he goes to rescue his father. They head to Naval HQ but a barrage hits and Cmdr. Newman is killed. An unexploded bomb lands, and the Project staff use the Tunnel to delay it long enough for the travellers to get clea

Tony and Doug arrive at the Siege of Troy, and are taken before Ulysses. Ulysses believes them to be gods, while his lieutenant Sardis doubts them. Doug wins a swordfight with Sardis to prove his divinity, so Sardis goes to Troy and tries to betray Ulysses and the Greeks. In the next battle the two travellers are almost overwhelmed, so the Project scientists send back Jiggs with a bag of hand grenades. Doug is captured during the next battle and put on the rack to be tortured. Jiggs is returned, but ages horrendously while trapped in the infinity of the Tunnel. The scientists manage to reverse the effects, but accidentally pull a Greek soldier forward (they manage to send him back). Tony ""inspires"" Ulysses to create the Trojan Horse, and goes inside it to rescue Doug. The ploy is successful. Ulysses kills Paris and rescues Helen, while Tony rescues Doug and the two vanish before the astounded Greeks.

Tony and Doug arrive on Devil's Island in March 1895, where they are taken into custody as escaped prisoners. Doug manages to escape and hold the Commandant at gunpoint to convince the man he is a stranded American, but the Commandant plays along until he can get the drop on him. The Project team plan a recovery which goes awry (again), hauling in the prisoner Boudaire. General Kirk tells Boudaire that they cannot escape with Captain Dreyfuss, one of the prisoners, because historically Dreyfuss never escaped at that time, so taking him along would prevent the success of any escape attempt. Boudaire is returned but is shocked by the experience and doesn't remember his instructions. Tony and Doug stage an escape but realize that they have been led to do so so that they can all be ""shot while trying to escape."" Boudaire recalls that Dreyfuss must remain. Tony and Doug attack the ambushing guards while the other prisoners escape, and then are whisked off by the Time Tunnel.

Tony and Doug arrive at the Alamo on March 6, 1836 - the day Santa Ana wiped out the defenders. The two make it to the fort, but are put into custody when Tony tries to convince the stubborn Col. Travis the fort is doomed. Tony escapes but is captured by the Mexicans. An injured Doug captures Capt. Reynerson and tries to convince him he can see the future. When Col. Travis is killed in a fall during Tony's escape as Doug predicted, Reynerson believes him. Tony manages to escape the Mexicans with the help of a sympathetic Dr. Armandez. Meanwhile, the project team have been trying to stage a recovery, but grab Col. Travis by mistake. He is stunned, but Kirk eventually convinces him by showing him future-footage of the battle, and Travis' own death. Convinced that he must let Doug go before the massacre begins, Travis returns and frees him. The Alamo defenders are gunned down, and Tony and Doug reunite. With Armandez' aid they get Reynerson's wife out before they are whisked away.

Doug and Tony show up in June 1215 when the Earl of Huntington is trying to convince King John to sign the Magna Carta. Doug is captured along with Huntington when the King gets surly and orders their torture. Tony manages to rescue Doug and they flee where they meet up with Huntington's men: Little John, Friar Tuck, and the Merry Men. Using their knowledge of modern-day chemicals, Doug and Tony help the rebels while trying to get hold of a homing post that the Project staff has sent back to them to enact a rescue. With the time travellers help Huntington and his men capture King John and force him to sign the Magna Carta...at Runnymede! The homing post fails to work and the two are swept off.

Tony and Doug arrive near the Italian-Austrian Alps on October 23, 1915, as the Germans are preparing to bombard the area. An explosion knocks them out and uncovers the stone coffin of Emperor Nero. Nero's sword floats out of it, but kills a German soldier. They awaken and discover they are beneath the villa of Count Galba. He claims the two travelers are American friends, covering for them against the invading Germans. The ghost of Nero has other ideas, and wants to kill Galba, whose ancestor was responsible for Nero's death. First a German soldier then Tony are possessed by the ghost. The Time Tunnel project staff manage to free Tony with a high-powered electrical discharge on the recommendation of a Dr. Steinholtz, whom they call in. However, they then pull the ghost into the current day, but manage to send him back. Tony and Doug get Galba to safety and help lead the Italian resistance to the Germans. The German leader, Neistadt, flees but Nero's ghost delays him long enough for an

Doug and Tony arrive in Lincoln in late April in the 1860s, and run afoul of Billy the Kid. Doug shoots Billy and apparently kills him, then he and Tony escapes. It turns out Billy took the bullet in the belt buckle, and he goes after the two. They manage to capture him thanks to a voice-transmitted distraction provided by Kirk, but Tony goes back into town and is mistaken for Billy. Pat Garrett arrives and eventually convinces the sheriff that Tony isn't Billy, but the mob isn't hearing it. Billy catches up to Doug and challenges him to a gunfight. Pat and Tony get involved, with Tony knocking out one of Billy's men planning to backshoot Doug and Pat intervening when Doug blows it. The two men are whisked off again.

Tony and Doug arrive in Khartoum, November 2, 1883, in the middle of a battle between British and Arab forces. However, two aliens take them prisoner. The aliens plan to conquer Earth with missiles, and will be ready to launch in two hours - they send a bomb to the Project to prevent them from interfering. Tony is taken to be killed but the alien with him is forced to teleport out when the British approach. Then both guys are transferred out of the cave by the Tunnel staff, where they make contact with Captain Henderson and convince him about the aliens. The three of them attack the cave and manage to stop the aliens, with some help when the Project send the bomb back to the aliens.