A military exercise is in progress on the bank of a deep river. The tanks will have to ford the water obstacle. A three-member crew - lieutenant Vladimir, sergeant Toma, and Gospodin, operates the tank. There is a lurking hostility among the three. Unfortunately, the engine goes out of action in midstream and the tank runs aground. The support party on the bank finds out that Gospodin has forgotten to take along the aqualungs. The crew spends seventy minutes in the tin box tomb. The water level gradually raises, the air runs out, the dome lamps go out. Their only link with the outside world is the bit of sky barely visible at the other end of the snorkel. The three make a summing-up and arrive at truths. The childish animosity, the injured self-esteem and the mistrust gave way to the lust for life and to the indispensability of mutual assistance.