WARNING, CONTAINS SPOILERS!!
Marble Hornets is a show created by Troy Wagner and Joseph DeLage, starring Troy Wagner, Joseph DeLage, and Tim Sutton. It is a paranormal show that revolves around Jay, who uploads his experience with an unknown entity known as the Operator (videos labeled as 'Entries'), who is stated to be a different entity than Slender Man, the creepypasta that inspired the making of Marble Hornets. The Marble Hornets Channel is where Jay uploads these entries - it should be kept in mind that another channel, under the name "Totheark" also uploads videos centralized on what Jay is investigating, sometimes even directly speaking to him. This article was written by Ian Walker.
Jay starts a YouTube channel entitled 'Marble Hornets', a film his friend Alex Kralie had been working on but had suddenly dropped. He recounts asking Alex for the tapes, in which Alex complies but requests that Jay never bring up the tapes to him again, and recommends he burns them. Jay, three years later watches the footage, notices that not all the tapes are from shooting Marble Hornets. Some tapes are from Alex, not shooting the movie and he's behaving very strangely in them. He notices in some there is a pale white figure (The Operator) and decides to look further into the matter. In the midst of all this, it can be confusing to know who is holding the camera. Throughout the rest of the series, Jay (being the protagonist) is the one primarily filming.
Later in the series, Jay finds Tim, an actor for Marble Hornets, and questions him about how Alex was behaving and if he ever saw anyone in a suit. Tim lies and says he doesn't know anything.
Even later in the series, it is obvious that Alex is not a trustworty source anymore, most likely being controlled, operated or influenced by The Operator himself. When with Jessica and Jay, Alex pretends to have found something but intends to kill them all in the woods instead. However, when at gunpoint, Alex is tackled by 'Masky', later found out to actually be Tim under some unknown influence or black out. Jay and Jessica escape, along eventually with Alex and Tim. This is the point in the series where Alex's whereabouts are unknown.
Continuing on, and skipping some, Alex returns to Tim's house in search of him, claiming that he is why this is all happening. He yells this and more in Tim's house, as Tim films from in a closet. Alex says that he guessed that, all along, it was his fault, but continued to say that it isn't, and that Tim needs to die for it all to be over. He sets Tim's house on fire.
Tim then ties up Jay and leaves him to go and search for Alex. However, Hoodie, (Brian) finds Jay tied up and throws him a knife to cut loose, and a camera to film. Jay watches as the hooded figure runs away.
Jay ends up going to Benedict Hall and getting shot by Alex in the gut/chest area, and whilst running into one of the rooms to try and hide, gets attacked by The Operator. It is assumed that The Operator killed Jay, as he was prone from being shot.
Tim finds Jay dead at his house, paper below him with the operator symbol and words such as, "Your fault." on the papers.
However, Tim running around Benedict Hall, finds the hooded figure and watches as he accidentally fell from the top floor and dies. Tim runs down to see who was behind the mask, and even though it was scratched out on video, it was Brian.
Tim finds and kills Alex, who states it wind end until he dies.
After a while, we can see Tim throwing away his mask in the garbage and meets Jessica, who went missing, at the hall. He says that Jay had gone and left this all behind him, but this is a lie since Jay is dead. (He most likely did it to protect her)
So, at the last entry, Tim is seen driving in his car with a bottle of pills in his hand. He states that 'everything is fine'. Whether or not he is dead is up to the viewer.
NOTE: READ AT YOUR OWN RISK. CONTAINS MAJOR SPOILERS TO MARBLE HORNETS