The Democratic presidential primary debate on Tuesday will feature Vermont Sen. Bernie Sanders' return to the campaign trail after a health scare and one fresh face.

The field of candidates expected to take the stage in Westerville, Ohio, is the largest to date, with a dozen qualifying under the rules set by the Democratic National Committee. The matchup includes billionaire activist Tom Steyer, who will be making his first appearance on a debate stage. It also features the return of Hawaii Rep. Tulsi Gabbard, who did not qualify for the September debate. After threatening to boycott Tuesday's debate, she reversed course Monday, saying she would attend after all.

Sanders, meanwhile, will take the stage two weeks to the day after he suffered a heart attackwhile campaigning in Nevada.

Here's everything you need to know about the fourth debate.

When and where is the Democratic debate?

The debate is being held at Otterbein University in Westerville, a suburb of Columbus. It's scheduled to begin at 8 p.m. ET Tuesday and is being co-hosted by CNN and The New York Times.

It will be moderated by CNN anchors Anderson Cooper and Erin Burnett and Times national editor Marc Lacey. It's slated to run three hours.

Who made the stage?

The 12 candidates who qualified by having both 130,000 individual donors and reaching at least 2 percent in four qualifying polls, in addition to Gabbard, Sanders and Steyer, are former Vice President Joe Biden; New Jersey Sen. Cory Booker; Mayor Pete Buttigieg of South Bend, Indiana; former Housing Secretary Julián Castro; California Sen. Kamala Harris; Minnesota Sen. Amy Klobuchar; former Texas Rep. Beto O'Rourke; Massachusetts Sen. Elizabeth Warren; and entrepreneur Andrew Yang.

Who's standing where?

As with the previous debates, stage position has been determined by polling averages, so front-runners Biden and Warren will be center stage. The overall order from right to left is Gabbard, Steyer, Booker, Harris, Sanders, Biden, Warren, Buttigieg, Yang, O'Rourke, Klobuchar and Castro.

There was some question over whether Gabbard would say aloha — or aloha.

She tweeted out a statement last week saying she was considering boycotting the debate because the DNC and "the corporate media" were "rigging the election" by using polling as qualifying criteria.

Gabbard did not qualify for the September debate because of polling, and so far has not hit the polling benchmarks for the next debate in November. In her tweet, she said the "so-called debates" are "not debates at all, but rather commercialized reality television meant to entertain rather than to inform or enlighten."

But Monday, she appeared to have a change of heart:

I will be attending the debate. — Tulsi Gabbard (@TulsiGabbard) October 14, 2019

What are the rules?

As was the case at the September debate, candidates will have 75 seconds to answer direct questions and 45 seconds for rebuttals, according to debate co-host The New York Times. There will be no opening statements.

How can I watch the debate?

The debate will air live on CNN and stream on cnn.com and nytimes.com. NBCNews.com will live-blog the debate throughout the night, offering live updates, fact checks and analysis.

When is Round 5?

The fifth debate is scheduled for Nov. 20 in Georgia, and will be hosted by MSNBC and The Washington Post.

To qualify for that stage, candidates have to meet fundraising and polling criteria laid out by the Democratic National Committee, and those benchmarks are higher than the previous debates. They call for candidates to hit at least 3 percent in four qualifying state or national polls, or 5 percent in two qualifying state polls one week before the debate. The fundraising threshold requires candidates to have received contributions from 165,000 unique donors, including 600 unique donors in 20 states.

An unofficial survey by NBC News shows eight candidates appear to have qualified to date — Biden, Booker, Buttigieg, Harris, Sanders, Steyer, Warren and Yang.