Interplay reckons it could end up developing the sixth Fallout game if Bethesda successfully prevents it from releasing Fallout Online.
Speaking exclusively to Eurogamer, Interplay president Eric Caen said his company was "ready to fight for years if necessary" in the high-profile legal battle with Bethesda over the future of the in-development post-apocalyptic MMO.
"We sold the Fallout IP to Bethesda in exchange for a certain amount of cash and the right to do the Fallout MMO," he said. "If they refuse to let us do the game, then the sale of the IP is terminated, and they will be allowed to do only one more Fallout, 5.
"But in that case, the IP will come back to us, and of course, we will complete our work and release Fallout MMO."
Caen's Fallout 5 refers to the next Fallout game from Bethesda, with last year's Fallout: New Vegas considered the fourth game in the hugely successful series.
"The original licensing deal was for three games and their DLC," Caen continued. "So they already did Fallout 3, then Fallout: New Vegas, and they can only do one more Fallout, 5, if the sale of the IP is cancelled by the court.
"We will love if we have to do Fallout 6 and sequels. But we will see what happens in court. It can be this year or later... We have the back-up of our shareholders to fund this fight."
Bethesda purchased the rights to the Fallout franchise from Interplay in 2004 for $5.75 million. It licensed the online rights back to the publisher before starting development on Fallout 3.
Interplay announced its intent to release a Fallout MMO soon after that deal was signed, with a release currently slated for 2012. But a legal tug of war over the future of the game has put its release in doubt.
The latest development saw Bethesda hit back at Interplay's claim that its attempt to derail its planned Fallout MMO was "absurd". Bethesda had claimed it gave Interplay rights over the Fallout trademark and nothing else, meaning it had no right to use essential Fallout ingredients such as weapon art and game concepts, for example the Pip Boy and what's called the "World Bible".
"We have confidence in justice, and we should win this case," Caen said. "But the decision isn't in our hands."
Interplay has so far refrained from showing much from Fallout Online because "anything we show will help Bethesda in their fight".
Caen refused to comment on his commitment to launch the game during the second half of 2012, but insisted the game was well into development.
"Since early 2009, we have a virtual Fallout world that exists and grows every day. We don't want to release too many elements because of the litigation. We don't know yet when we will reveal more. Sorry!
"Our team, led by Chris Taylor and Mark O'Green [two of the creators of the original Fallout], loves Fallout and what we are putting together. We all hope we will be able to show you soon more of our work, and that you will like it."
Bethesda continues to argue it owns all the Fallout IP rights.