This is a story related achievement which will unlock with natural story progression.
Shortly after talk to agent Humphries in the Police Station you will need to visit Lucy. You will be told that she doesn't know where The Captain is. If you talk to the boy (Wiggins) across the street from her house, he will tell you he is down the street.
After you find The Captain you will need to make your way to the Hospital and buy medication for The Captain. Once you return to Watson and hand over the medicine this achievement will unlock.