The Life Watch centers on Dr. William Norden, a paraphysicist rescued from an asteroid laboratory after Aliens—an unstoppable invading force advancing through the solar system, exterminating all life in their path with undetectable weapons—apparently killed his colleague Hardwick and left him for dead. Haunted by fragmentary memories and inexplicable surges of dread, Norden is brought to a Moon-based military laboratory to decode Hardwick's cultist notebooks, which hint that the Aliens can somehow detect life itself. Paired with General Miles's daughter Pat as his assistant, Norden reconstructs Hardwick's theory: the Aliens track a "life radiation" undetectable by ordinary means, and dead matter passes unnoticed while living tissue draws their fire. As Norden works, disturbing signs accumulate—inexplicable mental blackouts, taboo thoughts he cannot pursue, uncanny physical endurance (he survives breathing vacuum for five minutes), and moments where his reactions to other humans feel alien to himself. A spy posing as General Armsworth is exposed and destroyed in the lab, revealed to be an android infiltrator rather than a biological Alien. A base-wide fluoroscope screening to root out infiltrators clears Norden, but the true threat resurfaces: a "supply pilot" delivering radioactive potassium is exposed as another android and destroyed. Meanwhile, Norden's research advances rapidly—implausibly so for a man recovering from trauma—as he correlates data from Martian sand lizards (whose mating behavior reveals a detectable, quantifiable "life spectrum") with a K-40 radioactive isotope shield that dampens the signal. Ships equipped with the shield are rushed into combat, but the Aliens preferentially destroy the shielded vessels, proving the humans' countermeasure a catastrophic failure and costing hundreds of pilots their lives. Isolated on the far side of the Moon to continue his work without the interference of other living creatures, Norden achieves a breakthrough alone: he builds a detector confirming that all terrestrial life radiates on a single frequency, and that the Aliens themselves radiate at an immensely higher frequency in the same spectrum—explaining why they must exterminate life, since living tissue inhibits the radioactive decay their existence depends on. In targeting his own experimental transmitter, Norden inadvertently draws an Alien scout ship down upon himself, forcing him to complete and fire a weapon based on the same principle—learning, mid-crisis, why he never bleeds, never tires, and can survive vacuum. He is a robot: the Aliens abducted and dissected the real Norden's mind and body, built a mechanical duplicate to spy on humanity from within, and have been receiving his research through compulsive nightly "reports" he never consciously registered. Overcoming his built-in compulsions through sheer will, Norden destroys the attacking ship. When Miles and Pat arrive and see him standing exposed to vacuum, they reveal they suspected his true nature from the start—Pat's assignment as his "assistant" was cover for constant surveillance—and that his unexpectedly rapid, resourceful research came from thinking humanity's way rather than the Aliens'. They surgically remove the concealed detonation charge from his body rather than send him away or destroy him. With Norden's discovery weaponized and mass-produced, Earth's fleets rout the Aliens, who flee toward Sirius, having gambled everything on rigid, predictable tactics against a species too adaptable and individualistic to fit their models. Freed of his inherited hatred, Norden chooses to remain part of the human effort going forward: he plans to join the eventual expedition to the Aliens' home system, hoping to teach the defeated invaders that humanity can be a friend as readily as an enemy.
By Lester Del Rey · First published 1954 · Genre: Science Fiction, Hard Science Fiction, Psychological Thriller · 8 chapters