The leak claims that Halo: Infinite will have 25 missions and will take approximately 20 hours to complete the campaign. In addition, there will be 35 side missions that will add an additional 15 hours of play. During the main campaign, players will be Master Chief (not really a surprise as this has been largely confirmed by 343i). The other characters expected to return are: Arbiter, Atriox, Locke and Palmer.
The lessor reviews some rather juicy additional details. As we know, Halo 5: Guardians ended with Cortana becoming a thug, launching a plan to destroy everything. Apparently, the Master Chief captures Cortana and somehow sends her away to prevent the galactic civil war from happening. All of this puts an end to the campaign on the planet Reach where this time travel plot ends up restarting the Halo series.
With any leak, there is certainly a LOT of room for error or trolls, but it certainly seems possible.