Samarjit was an Indian who served in the British army during World War I. His fellow soldiers made fun of him and called him "Smith" because they couldn't pronounce his real name.
They volunteered him to accompany Dr Smith on a dangerous mission. Samarjit accompanied the Doctor through enemy lines while the Doctor searched for an undetonated bomb. They then found a structure that had been buried but was exposed during the fighting. The Doctor found something in the structure, then set the undetonated bomb to destroy the structure. Samarjit was hit by enemy fire, and when he woke, he was in hospital. He learned that the Doctor had given him elixir that healed his wounds.
Soon after the end of the war, Samarjit died from a cold. However, the elixir allowed his consciousness to live on. He was angry at the Doctor; they met once a year on Christmas Eve so the Doctor could convince him to let go. After almost a century, Samarjit was ready to let go. (PROSE: Last Christmas)