Future Diary: Paradox is a spin-off manga from the original Future Diary storyline featuring Aru Akise as the main character, and takes place in an alternative timeline when Yuno Gasai is accidentally killed while Yukiteru Amano gets heavily injured. It is unclear whether or not the events have happened in canon, but the possibility is likely since the events do not seem to contradict the continuity of the series.
Muru Muru accidentally makes Yuno disappear while she was making mochi. As a result, Yuno never showed up in time to prevent 3rd from attacking Yuki at the 14th floor of the building where 3rd was actually killed in the original storyline.
Muru Muru decides she must take Yuno's place in order to prevent the Causality Continuum to change a lot more than it already did. She comes just in time to stop 3rd from killing the already badly injured Yuki by sending Akise to battle him instead. Akise kills 3rd by evading his attack and letting him stab an electric cable thus electrocuting himself instead of killing Akise. Muru Muru then transports Yuki away and puts him in a kind of healing device, stating that he'll be healed within a few days.
Meanwhile, she and Akise have to team up, Akise representing Yuki, and continue the predestined events of the survival game. Thus they both find themselves cornered by Minene Uryuu at Yuki's school. However, in contrast to the original events, Akise sends Muru Muru through the minefield with a fake bomb tied to her back making Minene flee thinking that the bomb was real.
Afterward, in order to not alter the Causality Continuum too much, they head towards the Omekata Temple where Tsubaki Kasugano, the sixth diary owner, resides. The events have changed and instead of 6th, 12th, and 5th's parents getting killed like it was intended, the current cult leader, Funatsu, was tricked into revealing the death of the 6th's parents were caused by him. Muru Muru, now gets mad cause the events didn't work like they originally went, she leaves.
Yuki is nearly healed and time is about to reset, meaning everyone will have their memories erased. Akise tries to find Muru Muru's secret by entering Mecha Muru Muru's, a robot made by Muru Muru to replace her while with Akise, memory bank (the robot is an exact copy, meaning it has the real Muru Muru's memories). Akise confronts Yuki, who is brainwashed by Muru Muru, and is stabbed with a sword that will erase his memories. While the situation seemed hopeless 5th, 6th, both 7ths, and one of 8th's followers arrive and help Akise into the memory bank while they deal with Muru Muru. In the memory bank, Akise sees the moment when the Yuno from the first world killed her second world counterpart and had been given her future diary from Muru Muru.
Sadly, the effects of the sword erased Akise's memory, and time has been reset. Now, Akise is experiencing deja vu and has become interested in Yukiteru.