It follows ministers ordering improvements following dozens of services a day not running because of train crew shortages.
However, it marks a third attempt to improve performance after an improvement plan in 2016 and another last April.
The Scottish Conservatives condemned the latest scheme as a "targetless, toothless and meagre attempt to buy time".
ScotRail agreed the remedial plan - which First Minister Nicola Sturgeon said yesterday was its "last-chance saloon" - would be no instant fix.
It said there was "no time frame for specific improvements to cancellations".
However, Transport Scotland said if performance did not improve from its current "breach" level by May next year, operator Abellio risked having its franchise terminated five years early.
The plan reveals the staff training backlog at the heart of ScotRail's woes will not be cleared until May.
An additional 30 conductors will be recruited and trained by July and 55 more drivers by May next year.
But ScotRail is so short of drivers it has already approached retired drivers to return to the cab, as Scotland on Sunday revealed.
Three extra veteran diesel "High Speed Trains" will be added to the fleet "to provide extra resilience and support training".
ScotRail currently only has half of its awaited 26 High Speed Trains in service, which should all have been carrying passengers last year on inter-city routes.
In addition, all but three are still awaiting refurbishment, such as to replace old-fashioned manual doors with electric ones.
Passengers are still being plagued with cancellations, with nearly 40 trains not running today, not operating over their full route or with fewer carriages than planned.
This has largely been caused by a backlog of staff training on the High Speed Train fleet and new Hitachi-built electric trains because they are being delivered late, and due to industrial action last year.
ScotRail said its "increased focus on fleet reliability" also included Hitachi technicians and seven maintenance controllers to resolve faults.
The firm also pledged £500,000 a year for a "performance improvement fund" to "identify and implement changes".
Information screens will be upgraded at 16 stations.
A new post has also been created to spearhead a three-year "train crew resource plan".
The cost of the plan will be paid by ScotRail operator Abellio rather than come from its Scottish Government subsidy.
ScotRail Alliance (with Network Rail) managing director Alex Hynes said: “Improving the service our customers receive is the priority for everyone at ScotRail, working with Network Rail and key suppliers.
"I am confident this plan will deliver significant improvements.
“We have worked hard to identify specific areas to focus our efforts where they will have the most impact.
"The funds we have invested in this plan is a demonstration of our commitment to delivering the service our customers expect and deserve.”
Transport secretary Michael Matheson said: "Transport Scotland has entered into a remedial agreement with ScotRail on acceptance of their proposals to improve performance.
"I have instructed my officials to ensure robust measures are put in place to monitor progress very closely.
"I expect to see a continued upward trend of performance improvement."
Mr Matheson said he would update MSPs at the Scottish Parliament's rural economy and connectivity committee on Tuesday.
Scottish Labour said the plan admitted ScotRail's performance would be below target for at least another year.
Transport spokesman Colin Smyth said: “Only ScotRail could publish a performance improvement plan which admits it will still be breaching the franchise for another year and doesn’t say when it expects to hit the actual target.
"Since the SNP handed Abellio the ScotRail franchise in what they claimed was a world-leading deal we have seen two improvement plans and another two remedial plans.
"Nicola Sturgeon said this week Abellio was in the last chance saloon, but they have had more final warnings from the SNP than the Rolling Stones have had farewell tours.
"If the company were as good as running the railway as they are at running rings round SNP ministers, then passengers would be getting a world-class service at rock bottom prices.
"Instead we have overcrowded, overpriced trains that either run late or don't run at all.
"That's why Labour's plans for public ownership is winning support across the country."
Scottish Conservatives transport spokesman Jamie Greene said: “Any new investment in people or trains is welcome but the fact these additional staff have not yet been recruited means passengers could be waiting months or even years before seeing any tangible improvements.
“The new plan itself admits that despite its many warm words, buried away at the back of the report is an acceptance it will be at least May 2020 before ScotRail has any hope of exiting its breach of the moving annual average performance targets it is currently missing.
“Of course, we all want ScotRail to succeed, but far from being ‘last-chance saloon’ for ScotRail, this seems like a targetless, toothless and meagre attempt to buy time and will fill no-one with any confidence that things will drastically improve any time soon.
“The government has truly missed a trick here."