Theory:

     Today, however, was different. Tilloo’s father was enjoying a siesta at home and Tilloo managed to get hold of his security card. Then, evading his watchful mother’s eye, Tilloo made his way to the forbidden passage.
 
     A metal door barred his entry. But Tilloo had watched his father slip the magic card into a slot. He did the same... and the door opened noiselessly. A well-lit passage seemed to beckon him.
 
     Snatching the card which had come out of another slot in the wall, Tilloo started his march along a gentle upward slope—for the passage led from the underground habitat to the surface of the planet. Tilloo looked forward to seeing the sun (if it was daytime) or the stars (if it was night) about which he had heard and read so much.
 
     But alas, that wasn’t to be!
Explanation:

Later the narrator stated that after getting scolding's from his mother, Tilloo couldn’t stop him from finding out what was in the prohibited passage. On a fine day, while Tilloo’s father was having a short nap after eating his lunch at home, Tilloo decided to learn what was hidden in the underground passage. So he tried to get hold of his father’s security card without his knowledge. The security card was used to unlock the closed door.
 
shutterstock_1542607016.jpg
Tilloo’s father was enjoying a siesta at home
 
Tilloo, after getting hold of the security card from his father, later considered escaping from his mother’s watchful eyes. Tilloo managed to get away from his mother because she would not allow him to use that way and she would scold him for not obeying her words. Even though disobeying the mother was the wrong thing, his curiousness made him do so. Therefore he planned well and made his way to the prohibited route without drawing his mother’s attention.
 
access-1276873_1920.jpg
Security card

When he got close to the forbidden path, he saw a metallic door that stopped him from moving further. Tilloo, on the other hand, had seen his father accessing the security card to open the metallic door numerous times. Later Tilloo had inserted the card into the slot with that thought in mind. When the card was inserted inside the metallic door, it opened quickly and noiselessly.

Later, when the door got opened, he stepped into a well-lit tunnel. Then Tilloo noticed the magic card he slipped to one slot came through another slot in the wall. Later he took the card and moved gently towards an upward slope. The upward slope was the way leads him to reach the surface of the planet from the underground passage. While he was moving towards the upward slope, Tilloo desired to see the sun and stars closer. He was interested in seeing the stars and sun because he had read about them in his book and had never seen them before. Also, he knew that if it were daytime, he would probably see the sun and if it were night time he could see the stars. Unfortunately, in the end, what he expected did not happen. The reason behind the statement was Tilloo thought that it would be easier to look at the sun and stars, and sadly, his wish didn't come true. It meant that he did not see either sun or stars.
 
BeFunky-collage (30).jpg
Tilloo thought of seeing sun and stars
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
Siesta Rest or sleep taken after lunch, especially in hot countries
2
MarchTo walk in a military manner with a regular measured tread
3
Evading Escape or avoid, especially by cleverness or trickery
4
Beckon To move your hand or head in a way that tells someone to come nearer
5
Barred Closed or secured with a long rod or rigid piece of wood, metal, or similar material
Reference:
National Council of Educational Research and Training (2007). Supplementary. An Alien Hand - Jayant Narlikar (66-73). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.