Unusually for a game, The Sentinel doesn’t have a plot to speak of. A number of magazines included a story in their review, but nowhere in the official packaging is a back story mentioned.
When the game loads, the player is asked to input a landscape number from 0000 to 9999. After this, they are prompted for an 8 digit secret entry code, for every landscape except 0000, which didn't require an entry code to play.
Before the game starts, the player is shown an aerial view of the landscape. Each landscape looks like a greatly extended chess board, but with platforms of varying heights. The aerial view also shows the relative positions of the Sentinel (who stands on a tower at the highest point on the landscape) and its sentries. Thankfully, the first few levels didn't have any sentries, thus giving the player a slightly easier introduction to the nuances of playing the game. The Sentinel (and sentries in later landscapes) remain inactive until the player expends or absorbs energy.
The main concept of the game is all about energy. Trees and Boulders are dotted around each landscape. Boulders are worth two units of energy, whilst trees are only worth one. The robot controlled by the player are worth three. The player can rotate on the square they occupy and absorb the energy of any boulders or trees where they can see the square those objects are standing on. Therefore, to absorb energy from something else, the player has to be on the same level or higher than the object. To move, the player has to create a robot on another square (costing energy to do so) and then jump into the new robot shell. They can then turn around and absorb the old robot shell and continue as before, assuming something else doesn't start absorbing it first!
Once activated, the Sentinel and sentries also slowly rotate on the spot, scanning the landscape for squares which contain objects of more than one unit of energy. If they can clearly see such a square, the Sentinel or sentry absorbs the item's energy, 1 unit at a time. Therefore a robot becomes a boulder, and a boulder then becomes a tree. Although the player can absorb trees, the Sentinel and its cohorts are a little more environmentally friendly and leave the trees alone!
If the player's robot falls under the gaze of the Sentinel or the sentries, an alarm triggers and the robot's energy levels start to reduce as the energy is being sapped 1 unit at a time. The only way out is to move, either by quickly creating a new robot shell to jump into or by performing an emergency hyperspace to a random square somewhere else on the landscape. The drawback with that strategy is that the player has little chance of absorbing their old robot shell and they might still fall into the deadly gaze of the Sentinel! Hyperspace also cost three units of energy (the number required to create a new robot shell) and if the player doesn't have enough energy to complete the jump, then hyperspacing can destroy the robot and end the game!
The total amount of energy on each level remains constant, so if the player is losing energy then for each unit they lose, a new tree (worth one unit of energy) is created and randomly placed on the landscape.
If the player survives and successfully absorbs the Sentinel, then an 8 digit code is presented. This code is based on the number of the landscape completed and the amount of energy left after the player hyperspaces to the platform where the Sentinel was standing before it was absorbed by them. The landscape the player moves to next is based on how well they played the current landscape (how much energy that have at the end). This meants they didn't have to play all 10,000 landscapes to reach the end!
On the subject of the end, the only complaint some expert players had was that The Sentinel didn't actually end! After completing the final level, the game returns the player back to level 000000 but with the access code equivalent to the energy they have amassed.