“We are building a foundation for the transformation of television networks, so that they will be available on demand and on mobile devices with a seamless interface for search,” he said. “This is leading the way for everybody.”

With the new service, Time Warner and HBO have to tread carefully so as not to jeopardize their core businesses. Both HBO and the other networks owned by Time Warner, including TNT and TBS, receive billions of dollars from cable and satellite companies for their programming. Mr. Bewkes and Mr. Plepler argue that the new service is complementary to their existing business, appealing to those who refuse to pay for cable or satellite TV. Yet some cable and satellite executives complain that HBO Now has the potential to undercut their offerings. For some smaller cable operators, the cost of HBO Now is cheaper than the rate they charge for HBO packages.

“What we’re saying to our partners, old and new, is, ‘Join us, use us in going to get all those people in the consumer base who want HBO,’ ” Mr. Plepler said. “Why is that not a win-win-win-win for everybody? Nobody’s been able to give me a good answer to why it isn’t.”

A day after Mr. Plepler announced HBO Now, CBS unveiled its own streaming service, and one for Showtime, which CBS owns, is also in the works. CBS chief executive Les Moonves said the timing of his network’s announcement was a coincidence, but he did not have what Apple offered to Mr. Plepler months later: An invitation to introduce HBO Now on the same stage where Apple unveiled its new watch.

“I admired the theatricality of it,” Mr. Moonves said. “It made the announcement more important. It was show business. It got the attention they both wanted. I mean that positively. I was jealous.”

To the people at Apple, its first partner on HBO Now, the invitation was a no-brainer. “I think they have the best content on the planet,” said Eddy Cue, the senior Apple executive in charge of brokering deals with media companies.