- "In desperation, the world leaders turned to their last chance: an ancient artifact called the 'Time-Lock'. They believed this relic from a long forgotten race was a portal to a point in ancient history, and their only chance for survival. If only they knew how to turn it on."
- ―Narrator describing the Time-Lock in Serious Sam 3: BFE.[src]
The Time-Lock is basically a time machine. However, it is designed so that it can only teleport someone back to the date it was armed. It can accept organic and inorganic objects, but will usually strip off any large inorganic objects on someone when they use it. It appears to require an immense amount of power, as there are two generators dedicated to powering it, with one in Karnak and another one in Luxor.
It was created by an unknown species, but discovered by the Sirians while exploring the galaxy. During Mental's siege on the Sirians, they sent the Time-Lock to Earth, a planet they were just starting to explore, to prevent it from falling into Mental's hands. After the fall of Sirius, the last remaining Sirians decided to arm the Time-Lock to their time in case it was needed by the humans for whatever reason. After being armed, the Time-Lock was buried in the Temple of Hatshepsut, and it was lost for many, many generations.
A few millennia later, Earth was attacked by Mental. Like the Sirians, the humans were losing to the point where Mental only had to defeat Earth to complete the annihilation of humanity. The Earth Defense Force discovered the Time-Lock, and began making a last-ditch plan where they would insert a team via the Time-Lock in order to kill Mental in the past. However, the big problem was that the power generators were shut down, and their locations were unknown.
At the start of Serious Sam 3 BFE, Hermann Stein and his escort, Bravo Team, managed to find some clues to the generator's locations in some ruins found in a museum in Cairo but all his escorts were killed and he refused to hand the data to the HQ, so Alpha Team was send in to rescue the professor. Sam, the only surviving member of the Alpha Team, decided to rescue the professor alone, but was too late when he discover Stein's remains. However, Sam managed to find data of the clues and sent it back to HQ, then he left Cairo to get to the Great Pyramid.
Sam traveled to the Great Sphinx, which apparently lead to a clue. The recovered texts stated that there was something under it and that there was a way inside, but Sam took the easy way and blew the Sphinx up, which revealed the entrance to underneath the Sphinx. Sam found an old Sirian base and a hologram that showed where the Time-Lock's generators were and how to re-activate them. After he sent this info to HQ, Hellfire arrived to extract him and flew out to the first generator's location; Karnak.
After Hellfire dropped Sam off in Karnak, he activated the first generator that powered the Time-Lock. After that, he traveled to Luxor and activated the second generator there as well. When both generators were operational, Quinn informed Sam that he could return to headquarters, as his mission was complete and Charlie Team was ready to deploy. After Sam destroyed an Alcor Class Warship, Wilson arrived and extracted him.
Unfortunately, Quinn informed Sam that the Time-Lock failed to come online as they expected. Both of the generators were turned on but it hadn't started the Time-Lock because there was a safety switch (which was like a fail-safe) somewhere in Philae that prevented it from activating, and Wilson changed course to drop Sam in Philea to find it. After Sam fought his way through the enemy forces, he managed to activate the safety switch in the Tomb of Sethirkopshef and thus the Time-Lock was activated. After Sam defeated the Scrapjack Boss, Wilson came back for him and they both flew off to return to headquarters.
While returning to HQ, his helicopter was shot down by a Major Bio-mechanoid and he lost contact with HQ. After traveling through some temples, Sam finally managed to contact HQ again. However, it turns out that Mental had launched a sneak attack on HQ and killed Charlie Team, the group that was supposed to enter the Time-Lock, among other people.
With Charlie Team dead, Sam decided to go to the Time-Lock himself and start Charlie Team's mission. He took control of a car in a slum, then drove it to the Time-Lock until he ran out of gas. From there, he fought his way to the Time-Lock. As he reached the final stretch, he was attacked by Ugh-Zan IV. Thanks to some creative thinking, he managed to kill Ugh-Zan IV and enter the Time-Lock right before Earth was destroyed by Mental.
Sam was then warped to Ancient Egypt via the Time-Lock, and starts his journey to defeat Mental.
There is one huge inconsistency between the Serious Sam 3: BFE and Serious Sam: The First Encounter versions of the time-lock; its size. In Serious Sam 3, the Time-Lock is a large and tall machine that can safely fit multiple people in it and is buried underneath a part of the Temple of Hatshepsut. In addition, it is colored gold and dark blue and the four towers surrounding the time-lock are designed like Egyptian human shapes. In Serious Sam: The First Encounter, the time-lock is significantly smaller, is a grey color, located above ground, and its towers are not shaped like a figure seen in Egyptian statues.
In Serious Sam: Next Encounter, Mental warns the Evil Serious Sam Clone not to use his personal time-lock. However, the Clone ignores his advice and hops into it, which starts the problems in Next Encounter that Serious Sam has to fix.
In Serious Sam Advance, humanity has managed to adjust the time-lock so that they can warp anywhere in history. They use the time-lock both to explore the past and to make sure that Mental doesn't tamper with the past in any way. The game starts out with some agents from Earth disappearing during a routine time traveling trip. Sam is sent back into the past via the time-lock to see what happened.