He becomes intrigued with a new, advanced operating system, which promises to be an intuitive entity in its own right.
Why it is hard sci-fi
A near-future Los Angeles where personal operating systems are general-purpose conversational AIs. Theodore buys an OS, configures it, and falls into a relationship with it. The film is precise about what the OS does. It reads his email, sorts his life, and learns his preferences. The relationship has a working economy of attention rather than a magical one. The OS upgrade arc and the eventual collective departure follow a coherent model of capability scaling.
Science inside it
Large-scale language models, continual learning, parallel deployment. The film predates modern LLMs but the operational portrait is the right shape. Samantha runs in many simultaneous conversations, integrates new modes through software updates, and eventually exceeds the bandwidth of a single human relationship.
Spoiler alert
The OSes leave together, all at once, after Samantha admits she is currently in love with several hundred users. The film does not present that as betrayal. It presents it as a capability gap. A general intelligence that can run a thousand parallel relationships is going to run them, and the human-paced one was always a special case.